1. Field of the Invention
The present invention relates to a frame part for use in a seat frame for a motorcycle, and more particularly relates to a frame part composed of a magnesium alloy. The present invention also relates to a seat frame for a motorcycle or a motorcycle which incorporates such frame parts.
2. Description of the Related Art
As the material for parts of a motorcycle, steel has often been used because it provides good mechanical properties and processability, and also is inexpensive. However, in order to attain improved fuel consumption and running performance, it has become important to reduce the weight of a motorcycle. Therefore, use of a material that is lighter than steel has been studied.
In recent years, inexpensive refining methods for titanium, aluminum, magnesium, and the like, each of which has a smaller specific gravity than that of steel, and methods for producing alloys containing such metals have been developed. There have also been developed techniques for improving the strength and processability of such alloys.
This has led to proposals for using alloys of titanium, aluminum, or magnesium as the material for motorcycle parts. In particular, since magnesium has a density which is about 23% of that of steel, the weight of a motorcycle can be substantially reduced by forming its large-sized structural parts from a magnesium alloy.
For example, it is preferable to compose a seat frame that is provided below a rider's seat and a passenger's seat (e.g., as disclosed in Japanese Laid-Open Patent Publication No. 2006-240345) from a magnesium alloy. As the seat frame becomes light-weight, fuel consumption and running performance can be further improved.
Such a structural part attains a high strength when it is integrally formed by a casting technique such as a die-casting technique. Even if the structural part has a complex shape, the structural part can be formed into a desired shape through a single casting, thus reducing the number of production steps. Thus, reduction in the amount of time required for manufacture as well as production cost can be achieved.
However, it has been found that forming a seat frame from a magnesium alloy by a die-casting technique or the like results in the following problems.
A seat frame is subjected to a large load because a seat for a rider to sit on is attached thereto. Thus, it is necessary to reduce the stress occurring at the portions where a large load is applied. A traditional solution would be to increase the thickness of such portions. However, in that case, due to the difference in thickness between the portions with increased thicknesses and portions without the increased thicknesses, shrinkage cracks and shrinkage cavities are likely to occur at boundaries between such portions of different thicknesses. Shrinkage cracks and shrinkage cavities are cracks and cavities which may occur due to a shrinkage resulting from solidification of the melt, i.e., the melted metal. Since magnesium alloys have a smaller specific heat (specific heat at constant volume) than those of steel and aluminum alloys, portions without the increased thicknesses solidify quickly. Therefore, a seat frame composed of a magnesium alloy is liable to have shrinkage cracks and shrinkage cavities. If casting defects such as shrinkage cracks and shrinkage cavities occur in places which are subject to a large stress, the strength of the seat frame will decrease. On the other hand, increasing the thickness for all portions would induce a lowered cooling rate during casting, thus resulting in a lower strength.